Disposing two substrates at a regular spacing forms an air layer which is a void space between the substrates. The air layer formed between the substrates serves as a low refractive index layer that reflects light entirely, for example. Thus, for example, in the case of an optical film, components such as a prism, a polarizing film, and a polarizing plate are disposed at regular spacings to provide air layers each of which serves as a low refractive index layer between the components. Forming air layers in such a manner, however, requires disposing the components at regular spacings, which does not allow the components to be stacked sequentially and causes time and trouble in production.
For solving such problems, there are attempts to develop a member such as a film having a low refractive index as a substitute for an air layer which is a void space between the components. Regarding such a member, as an example of achieving both a high porosity and a high strength, there is a method of applying the member to an antireflection layer of a lens (for example, see Patent Documents 1 to 4). In this method, a layer with void spaces (hereinafter, also referred to as a “void-provided layer”) is formed on a lens and then baked at a high temperature of 150° C. or more for a long time. This method has the problem that the thus obtained void-provided layer is inferior in flexibility. On the other hand, there is an example of application of a void-provided layer which has not been subjected to baking treatment (for example, see Non-Patent Document 1). This method has the problem that the obtained void-provided layer is inferior in film strength and cannot impart an impact resistance.
There have been examples disclosing a method of forming a silica aerogel film on a long resin support (for example, see Patent Documents 5 and 6). However, the silica aerogel film obtained by this method has a refractive index of more than 1.30, which cannot be a substitute for an air layer by any means.
On the other hand, as a light-emitting device, a flexible device having flexibility receives attention nowadays (Patent Documents 7 and 8).